1. Field of the Invention
The present invention generally relates to a control apparatus for a synchromesh type automatic transmission system of an internal combustion engine mounted on a motor vehicle, which is designed for automatically changing over speed stages of a synchromesh type multi-stage speed change gear unit incorporated in the automatic transmission system. More particularly, the present invention is concerned with the control apparatus for the synchromesh type automatic transmission system which is designed for suppressing effectively shock and gear squeaking phenomena which may take place in accompanying successive speed downshifts or speed changes from high to low speed stage upon application of sudden braking.
2. Description of Related Art
The control apparatus for the synchromesh type automatic transmission system such as mentioned above is known as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 270252/1988 (JP-A-63-270252). In the automatic transmission system known heretofore, transmission of the output torque or power of an engine such as an internal combustion engine to a gear-mesh type multi-stage speed change gear unit is controlled through on/off operation of an electromagnetic clutch.
In the synchromesh type automatic transmission system disclosed in the publication cited above, the speed stage (or speed gear) changeover operation is realized by driving or actuating correspondingly a three position hydraulic cylinder dedicated for selecting a speed stage and another three position hydraulic cylinder which is dedicated for shifting the multistage speed change gear unit to the selected speed stage.
More specifically, through combination of operations of a pair of electromagnetically controlled hydraulic values, a desired or target speed stage is selected by driving correspondingly the selection-dedicated three position hydraulic cylinder, whereon the multi-stage speed change gear unit is shifted to the selected speed stage with the aid of the shift-dedicated three position hydraulic cylinder which is actuated through the medium of another pair of electromagnetically controlled hydraulic valves.
In the speed stage changeover operation for changing the speed of a motor vehicle, the speed stage suited for the running state of the motor vehicle is determined by referencing a shift pattern set previously for the multi-stage speed change gear unit, as is well known in the art.
However, the conventional automatic transmission system suffers a problem that when braking is applied abruptly or suddenly (i.e., upon application of sudden braking) in the state where the motor vehicle is running at a high speed, e.g. at a fourth or fifth speed, the time taken for traversing a downshift pattern curve becomes short, as a result of which the downshift operation (i.e., shift operation from high to low speed stages) will take place successively. In the worst case, a succeeding downshift operation will occur before a preceding downshift operation has been completed.
Besides, because the sudden braking state mentioned above will remain effective continuously throughout the downshift operation, rotational synchronization will be established among the individual gears of the multi-stage speed change gear unit at an earlier time point when compared with ordinary downshift operation performed in response to manipulation of a shift lever by a driver of the motor vehicle, as a result of which uncomfortable phenomenon such as gear squeaking may take place.
As is apparent from the foregoing, the conventional synchromesh type automatic transmission system suffers the problem that when the sudden braking is applied in the state where the motor vehicle is running at a high speed, the downshift operation is effectuated successively, being accompanied with the gear squeaking phenomenon, as a result of which comfortableness of driving the motor vehicle is significantly impaired.
In the light of the state of the art described above, it is an object of the present invention to provide a control apparatus for a synchromesh type automatic transmission system of a motor vehicle which can effectively suppress occurrence of the gear squeaking phenomenon and shock brought about by the speed change downshift operation which may successively be effectuated upon application of sudden braking.
In view of the above and other objects which will become apparent as the description proceeds, the present invention is directed to a control apparatus for a synchromesh type automatic transmission system of a motor vehicle, which apparatus includes a shift/select actuator for driving a multi-stage speed change gear unit of a synchromesh type, a brake switch serving as a sensor for detecting a state where a brake pedal is depressed, an accelerator pedal position sensor for detecting a depression stroke of an accelerator pedal as an accelerator pedal depression stroke, an output rotation speed sensor for detecting a rotation speed of an output shaft of the multi-stage speed change gear unit as a vehicle speed, and a speed stage control means for making decision as to speed change on the basis of detection signals outputted from the sensors and controlling the shift/select actuator in dependence on the result of the decision, wherein a plurality of speed stages of the multi-stage speed change gear unit are automatically changed over from one to another. In the control apparatus for the synchromesh type automatic transmission described above, it is proposed according to a general aspect of the present invention that the speed stage control means is comprised of an acceleration/deceleration arithmetic means for determining arithmetically acceleration and deceleration of the motor vehicle on the basis of the vehicle speed, and a deceleration discriminating means for comparing the deceleration with a predetermined deceleration value, wherein when it is decided that the brake switch is in a closed state (on-state) and that the deceleration is not greater than the predetermined deceleration value, the multi-stage speed change gear unit is shifted to a neutral position (neutral state) and thereafter the decision as to the speed change is suspended.
By virtue of the arrangement described above, there can be implemented the control apparatus for the synchromesh type automatic transmission which is capable of suppressing gear squeaking phenomenon and shock which will otherwise take place in accompanying the speed-stage downshift operation upon application of sudden braking.
In a preferred mode for carrying out the invention, the speed change control means may be so designed that when it is decided that the brake switch is in the opened state (off-state) or the vehicle speed is zero, the speed stage control means clears the speed change decision suspend state to thereby allow the multi-stage speed change gear unit to be shifted to the speed stage determined on the basis of the shift pattern in dependence on the accelerator pedal depression stroke and the vehicle speed.
Owing to the arrangement described above, there can be implemented the control apparatus for the synchromesh type automatic transmission which is capable of restoring the automatic speed control conforming to the accelerator pedal depression stroke as soon as the sudden braking is released.
In another mode for carrying out the invention, the speed change control means may preferably be so designed that when it is decided that the brake switch is in the closed state (on-state) and that the deceleration is not greater than the predetermined deceleration value, a sudden braking application flag is set, wherein values of the sudden braking application flag in a preceding cycle and a current cycle, respectively, are referenced and when it is detected that the value of the sudden braking application flag has changed from a reset state to a set state, then a neutral control flag for shifting the multi-stage speed change gear unit to the neutral position and holding the multi-stage speed change gear unit in a neutral state is set, and wherein when it is detected that the value of the sudden braking application flag has changed from the set state to the reset state, a reset flag for allowing the multi-stage speed change gear unit to restore an ordinary speed change control operation from the neutral state is set.
With the arrangement described above, there can be implemented the control apparatus for the synchromesh type automatic transmission which is capable of suppressing the successive speed-stage downshift operation upon application of sudden braking and which can restore the automatic speed control operation rapidly upon releasing of the sudden braking.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.